Will a .21 engine fit into a 1/10 scale car? How much bigger than the .18 are they?

Yes you can put a .21 in a 1/10 scale...it can even be standard...

Perhaps if you can be more specific...what car are you wanting to put a .21 into?

Here are two typical examples of a big block and small block engine. Typically, .10, .12, .15, .18, and .20 small block engines use a smaller crankcase. Typical big blocks (.21, .26, .28, .32, etc.) use a larger block which is longer and wider than a Smallblock. So in most cases, a Smallblock to big block conversion requires a different mount and usually a different flywheel and clutchbell. Pictured is a Traxxas 2.5R and Novarossi BX21 (Ron Paris modified).

In some rare cases, a Smallblock mount can be modified to fit a big block, but mounting the engine is one thing. Accounting for the longer case and crank and throttle linkages is a different thing. Only a rare few manufacturers make a .21 displacement in a Smallblock case. Typically these engines are over-bored and generally won't run the same rpm and make the same power as a larger crankcase engine will.

That engine looks so clean 1Qwk...glass cabinet sort of thing.

Which one? LOL. The Novarossi is a $400 NIB rarity that sits in its box waiting for a new owner to buy it. It was the only big block I have that wasn't in a model. That poor 2.5R is awaiting some punishing break-in time if I can ever get it started. I pinched the sleeve so tight, it might need a bit of the blow torch to start.

it can even be standard.

Which one?...the Nova of course.

You not going to use it then?...bah

Nothing to put it in. Need a bump box for one, then something to put it in. Figured it's worth more in collector value than putting it in a basher. It was pretty expensive.